I changed the doveadm config per the migration guide:
service doveadm { inet_listener http { port = 12354 } } doveadm_password = redacted
Of course, I also added the doveadm service port to the exposed ports on the Docker container, and added the doveadm_password to that dovecot's config.
I then ran the "doveadm backup" command on the host, where the old dovecot instance is running, connecting to the doveadm service in the Docker container:
$ sudo doveadm -D -v backup -u myuser@mydomain.net -m MyFolder tcp:127.0.0.1:12354
which produces the following error:
Oct 23 21:27:58 doveadm(myuser@mydomain.net): Error: doveadm server sent invalid handshake: HTTP/1.1 400 Bad Request Oct 23 21:27:58 doveadm(myuser@mydomain.net): Error: Disconnected from remote: EOF
Meanwhile, the logs from the dovecot instance running in the Docker container show this error:
Oct 24 01:27:58 doveadm(192.168.65.1): Info: http-server: conn 192.168.65.1:26654 [1]: Client sent invalid request: Unexpected character <0x09> in request method
Any ideas as to what's wrong?